It is known to use a windmill type of mechanism coupled to a generator to provide electrical power as the wind rotates the blades, sails, or vanes of the windmill. However such conventional axial flow windmill mechanisms are costly. They need to be mounted high above the ground on a substantial standing structure to allow the blades to rotate. Such horizontal axis windmills also require constant adjustment so as to maintain their direction into the wind. Their speed needs to be governed for safety, but they are still vulnerable in storm conditions even when stopped. They are subject to fluctuating loading in the blades leading to fatigue stresses, caused by the effective wind speed being different at the top and bottom of their rotative cycle, and interference by the supporting structure.
A number of attempts have been made to design a more cost-effective wind powered generator. Thus, it has been proposed to provide two semi-circular vanes, rotationally mounted directly on the ground about a vertical axis so that each vane overlaps the other at the axis by about one third of their diameter so as to create a general letter "S" cross-sectional profile. Such a device is generally known as a Savonius turbine and, though it is simple to construct, support and operate as a self-starting turbine which is independent of the wind direction it has a relatively low output efficiency.
Attempts have been made to improve the efficiency by increasing the number of semi-circular vanes arranged about the vertical axis. However, the addition of a third or other blades does not significantly increase the efficiency of the arrangement although it remains self-starting.
There is an improvement in efficiency with the Darrieus rotor, invented by a Frenchman of that name in 1927. This turbine consists of three half rings of aerofoil section rotatably mounted top and bottom about a vertical axis. The principle of operation of the Darrieus rotor is the use of vanes each of which forms one half of a Torneau ring which has negative drag. Such an arrangement is efficient but is not self-starting, and requires running up to a critical speed at which it is self sustaining. A Savononius turbine is often mounted on the axis of a Darrieus rotor to provide the self-starting feature. Alternatively a motor is required to start the Darrieus turbine, and this adds to the costs.
From work done by Dr P. Musgrove and his students at Reading University it is known to provide two vertical symmetrical section vanes set on opposite ends of an horizontal arm mounted on a vertical axis and operating on the general principle of the Darrieus. Such an arrangement is not self-starting and it has other disadvantages, the main one being that the rotor varies in speed at different points in its cycle causing fluctuating current output and stress loads on the vanes, bearings and structure.
The safety factor with efficient turbines becomes significant for higher wind speeds. Special precautions need to be taken to prevent the disastrous accidents which have occurred with windmills and known turbines in storm or hurricane force winds. Generally powerful braking is required to limit the rotational speed even when a feathering control is applied to the blades or vanes.
In summary, the factors which should be considered in the provision of a commercially viable machine are:
(1) the need to avoid tall substantial structures to support the turbine; PA1 (2) the need to react to the wind over a wide range of wind speeds with a substantially constant power output; PA1 (3) the need to react to the wind regardless of direction so avoiding continual re-orientation to ensure that the windmill always points into the wind; PA1 (4) the need to reduce manufacturing costs and increase power output efficiency; PA1 (5) the need to eliminate the use of a step-up gearbox to drive a generator at an efficient speed by a slow turning rotor; PA1 (6) the need to be self-starting; and PA1 (7) the need to reduce excessive stresses generated in the structure at high wind speeds.
It is an object of the present invention to provide a cost-efficient wind powered turbine which is self-starting, substantially independent of wind direction and speed and which can be mounted on a low tower about a vertical axis of rotation.
It is a further object of the present invention to reduce the need for a gearbox by enabling the turbine to be connected to a large diameter, slow turning generator.